The present invention relates to a method of manufacturing nonoriented electrical steel by providing an ultra-rapid anneal to improve the core loss and the magnetic permeability.
Nonoriented electrical steels are used as the core materials in a wide variety of electrical machinery and devices, such as motors and transformers. In these applications, both low core loss and high magnetic permeability in both the sheet rolling and transverse directions are desired. The magnetic properties of nonoriented electrical steels are affected by volume resistivity, final thickness, grain size, purity and the crystallographic texture of the final product. Volume resistivity can be increased by raising the alloy content, typically using additions of silicon and aluminum. Reducing the final thickness is an effective means of reducing the core loss of restricting eddy current component of core loss; however, reduced thickness causes problems during strip production and fabrication of the electrical steel laminations in terms of productivity and quality. Achieving an appropriately large grain size is desired to provide minimal hysteresis loss. Purity can have a significant effect on core loss since dispersed inclusions and precipitates can inhibit grain growth during annealing, preventing the formation of an appropriately large grain size and orientation and, thereby, producing higher core loss and lower permeability, in the final product form. Also, inclusions will hinder domain wall movement during AC magnetization, further degrading the magnetic properties. As noted above, the crystallographic texture, that is, the distribution of orientations of the crystal grains comprising the electrical steel sheet, is very important in determining the core loss and, particularly, the magnetic permeability. The permeability increases with an increase in the {100} and {110} texture components as defined by Miller's indices since these are the directions of easiest magnetization. Conversely, the {111}-type texture components are less preferred because of their greater resistance to magnetization.
Nonoriented electrical steels may contain up to 6.5% silicon, up to 3% aluminum, carbon below 0.10% (which is decarburized to below 0.005% during processing to avoid magnetic aging) and balance iron with a small amount of impurities. Nonoriented electrical steels are distinguished by their alloy content, including those generally referred to as motor lamination steels contaning less than 0.5% silicon, low-silicon steels containing about 0.5% to 1.5% silicon, intermediate-silicon steels containing about 1.5 to 3.5% silicon, and high-silicon steels containinag more than 3.5% silicon. Additionally, these steels may have up to 3.0% aluminum in place of or in addition to silicon. Silicon and aluminum additions to iron increase the stability of ferrite; thereby, electrical steels having in excess of 2.5% silicon+aluminum are ferritic, that is, they undergo no austenite/ferrite phase transformation during heating or cooling. These additions also serve to increase volume resistivity, providing suppression of eddy currents during AC magnetizatin and lower core loss. Thereby, motors, generators and transformers fabricated from the steels are more efficient. These additions also improve the punching characteristics of the steel by increasing hardness. However, increasing the alloy content makes processing by the steelmaker more difficult because of the increased brittleness of the steel.
Nonoriented electrical steels are generally provided in two forms, commonly known as "fully-processed" and "semi-processed" steels. "Fully-processed" infers that the magnetic properties have been developed prior to fabrication of the sheet into laminations, that is, the carbon content has been reduced to less than 0.005% to prevent magnetic aging and the grain size and texture have been established. These grades do not require annealing after fabrication into laminations unless so desired to relieve fabrication stresses. Semi-processed infers that the product must be annealed by the customer to provide appropriate low carbon levels to avoid aging, to develop the proper grain size and texture, and/or to relieve fabrication stresses.
Nonoriented electrical steels differ from grain oriented electrical steels, the latter being processed to develop a highly directional (110)[001] orientation. Grain oriented electrical steels are produced by promoting the selective growth of a small percentage of grains having a (110)[001] orientation during a process known as secondary grain growth (or secondary recrystallization). The preferred growth of these grains results in a product with a large grain size and extremely directional magnetic properties with respect to the sheet rolling direction, making the product suitable only in applications where such directional properties are desired, such as in transformers. Nonoriented electrical steels are predominantly used in rotating devices, such as motors and generators, where more nearly uniform magnetic properties in both the sheet rolling and transverse directions are desired or where the high cost of grain oriented steels is not justified. As such, nonoriented electrical steels are processed to develop good magnetic properties, i.e., high permeability and low core loss, in both sheet directions; thereby, a product with a large proportion of {100} and {110} oriented grains is preferred. There are some specific and specialized applications within which nonoriented electrical steels are used where higher permeability and lower core loss along the sheet rolling direction are desired, such as in low value transformers where the more expensive grain oriented electrical steels cannot be justified.